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Q108309: FIX: Posting WM_COMMNOTIFY Messages May Fail

Article: Q108309
Product(s): Microsoft Windows Device Driver Kit
Version(s): 3.1
Operating System(s): 
Keyword(s): 
Last Modified: 03-NOV-1999

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The information in this article applies to:

- Microsoft Windows Device Development Kit (DDK) for Windows, version 3.1 
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SYMPTOMS
========

When using WM_COMMNOTIFY messages in a communications application, these
messages may appear to have stopped being posted.

CAUSE
=====

When COMM.DRV calls PostMessage(), it does not check the return value. Even if
PostMessage() fails, it appears to COMM.DRV that the message was sent.

For CN_EVENT notifications, the event must be cleared using GetCommEventMask()
before a new CN_EVENT notification will be sent. If the application never
receives the CN_EVENT notification, the event never gets cleared, so COMM.DRV
will not send a new CN_EVENT notification. If this happens, the notification
process appears to have stopped.

Similarly, for CN_RECEIVE and CN_TRANSMIT notifications, if these notifications
are not received, the application may not know to do a ReadComm() or
WriteComm(). If ReadComm() or WriteComm() is never called, the character level
will never cross the buffer threshold to create a new notification. For more
information on how CN_RECEIVE and CN_TRANSMIT notifications are generated, query
on the following words in the Microsoft Knowledge Base:

  " docerr and CN_RECEIVE and events and generated " (without the quotation
  marks)


RESOLUTION
==========

Implement a polling strategy to determine communications status, in addition to
using WM_COMMNOTIFY messages. This could be implemented in the message loop of
the application, or by using a timer.

STATUS
======

Microsoft has confirmed this to be a problem in Windows version 3.1. This bug
has been corrected in later versions of Microsoft Windows.

Additional query words: 3.10 buglist3.10

======================================================================
Keywords          :  
Technology        : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK310
Version           : :3.1
Solution Type     : kbfix

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